Technical note on boundaries

There were many changes in the tract boundaries used by the Census Bureau for the Chicago area between 1980 and 1990. Some outer counties were "tracted" for the first time. Many tracts were split. And a small number of tract boundaries were changed in more complicated ways.

Where tracts were split, we have used 1980 tract boundaries in these maps. 1990 data have been consolidated to 1980 tract boundaries.

We have generally ignored extremely minor changes in tract boundaries; usually, these involve a transfer of only a few dozen square meters.

In two parts of the urban area, tract boundaries in 1990 correspond poorly with tract boundaries in 1980. In these cases, we have created "supertracts" incorporating several 1980 tracts and several (different) 1990 tracts. Census data for both 1980 and 1980 have been consolidated to the supertract boundaries.